The invention relates to the field of voltage source converters, such as multilevel converters. Voltage source converters (VSC) have changed power transmission and distribution and using power electronics including semiconductor switching elements that can be turned off, such as IGBTs (Insulated Gate Bipolar Transistors) have found great use for DC transmission, reactive power compensation, control of active as well as reactive power, being able to create AC voltage out of DC voltage by means of switching control, and for converting AC to DC etcetera.
The multilevel converter technique, employing switching cells having respective energy storage elements for providing many voltage levels, wherein each voltage level being individually switched, can be used to compensate for example for reactive power in AC transmission networks. Often, the energy storage elements used consists of capacitors but may also be batteries.
The chain link H-bridge is a successful topology in the market and it is provided by several manufacturers. The H-bridge cells can be comparably simple and are suitable for being provided as modules, which helps to keep the costs of the construction down. Since the number of modules or switching cells is proportional to the AC voltage the system is easily scalable. Several variants of the chain-link topology exist, such as wye- or delta-connected H-bridge, and wye-connected M2LC.
FIG. 9 is a simplified example of a multilevel H-bridge converter according to the prior art, which converter is wye connected. The multilevel converter arrangement comprising three phase legs 10, 20, 30, each having a connection 15, 25, 35, respectively, adapted for connecting the converter arrangement to a respective phase A, B, C of a three-phase AC power transmissions grid, which transmission grid connection 15, 25, 35 is provided at a first end of each phase leg 10, 20, 30. A coil 13, 23, 33 is arranged at the transmission grid connections to smoothen the wave forms created by the converter. At the second end of each phase leg 10, 20, 30, the phase legs 10, 20, 30 are interconnected in the wye connection. Each phase leg 10, 20, 30 comprises a number of series connected switching cells 11A-n, 21A-n, and 31A-n. Each switching cell 11A-n, 21A-n, and 31A-n comprises an energy storage element 12A-n, 22A-n, 32A-n in the form of a capacitor. Each switching cell 11A-n, 21A-n, and 31A-n of the phase legs 10, 20, 30 are provided as modules of the same type as every other switching cell 11A-n, 21A-n, 31A-n; having the same bridge topology, the same type of capacitor and the same type of semiconductor switches. The multilevel converter also includes a controller 50 arranged for monitoring currents and voltages and controlling switching of the switching cells 11A-n, 21A-n, 31A-n. The controller 50 is provided to control the switching cells 11A-n, 21A-n, 31A-n to adjust the active and reactive power in a transmission line or grid. The controller 50 controls the switching cells 11A-n, 21A-n, 31A-n for control periods at a switching frequency that is substantially higher than the transmission grid frequency. Each phase voltage is adjusted for each control period, wherein the phase voltage is adjusted in accordance with a voltage reference signal that the converter receives as input from an outer controller provided in the transmission grid. The phase legs 10, 20, 30 may become unbalanced such that the voltages of some, or all, of the capacitors (energy storage elements 12A-n, 21A-n and 31A-n) deviate from the desired nominal cell voltages.
Cost and losses for such and similar converters are related to the total silicon area used in the converter. The silicon area is dependent on the voltage and current rating. A problem that may arise is that voltages over individual energy storage elements become too large or too low. Also, the total voltage available in the cells of a phase leg may for example become too low. For normal operation of the converter in industrial applications the designer must take into account the unbalance in the three-phase load. Unbalanced loads produce negative sequence currents that need to be compensated by the converter. To counteract the effect of these currents the total number of series connected cells will have too be large enough to compensate for the unbalanced condition due to the zero sequence voltage that needs to be injected. This has lead to the need for over-rated converters, wherein a number of extra switching cells has to be included in the phase legs for redundancy.
Using more switching cells is disadvantageous since the extra switching cells add to the costs of the converters and add losses during use of the converters.